文摘
We present a study on the gas-phase reaction of deprotonated cysteine with the lowest electronically excited state of molecular oxygen O2[a1螖g], including the measurement of the effects of collision energy (Ecol) on reaction cross sections over a center-of-mass Ecol range from 0.1 to 1.0 eV. Deprotonated cysteine was generated using electrospray ionization, and has a carboxylate anionic structure (HSCH2CH(NH2)CO2鈥?/sup>) in the gas phase. Three product ion channels were observed. The dissociation of HSCH2CH(NH2)CO2鈥?/sup> to NH2CH2CO2鈥?/sup> and neutral CH2S has the largest cross section over the entire Ecol range. This product channel is driven by the electronic excitation energy of 1O2 (the so-called dissociative excitation transfer), and is strongly suppressed by Ecol. Two minor channels correspond to the formation of HSCH2C(NH)CO2鈥?/sup> + H2O2 via abstraction of two hydrogen atoms from HSCH2CH(NH2)CO2鈥?/sup> by 1O2, and the formation of OSCH2CH(NH2)CO2鈥?/sup> radical via elimination of 路OH from an intermediate complex, respectively. Density functional theory calculations were used to locate various complexes, transition states, and products. Quasi-classical direct dynamics trajectory simulations were carried out at Ecol = 0.2 eV using the B3LYP/4-31G(d) level of theory. Trajectory results were used to guide the construction of a reaction coordinate, discriminate between different mechanisms, and provide additional mechanistic insights. Analysis of trajectories highlights the importance of complex mediation at the early stages of all reactions, and suggests a partially concerted mechanism for H2O2 elimination.